feat: add missing fields for stage instance

[Soheab]

Summary

This PR adds some missing fields to stage instance.

Adds the following to StageChannel.create_instance:

• send_start_notification
• scheduled_event

And to StageInstance:

• scheduled_event_id
• scheduled_event

And to HTTPClient.create_stage_instance's valid_keys:

• send_start_notification
• guild_scheduled_event_id

And to StageInstance (types/channel.py):

• guild_scheduled_event_id

Relevant API Docs:
https://discord.com/developers/docs/resources/stage-instance#create-stage-instance
guild_scheduled_event_id was added recently: discord/discord-api-docs@67ce08b to create

Fixes #934

This is a Code Change

• I have tested my changes.
• I have updated the documentation to reflect the changes.
• I have run task pyright and fixed the relevant issues.